AbstractThe vitrification effect during the curing reaction of amine terminated butadiene acrylonitrile copolymer‐(ATBN)‐modified epoxy resin is studied by the differential scanning calorimetry. A new reaction kinetic equation is derived to describe the effect of vitrification on the reaction rate at both glassy and rubbery state of reactants. ATBN is added to improve the impact properties of the cured epoxy resin. The phase separation behavior during the curing reaction is analyzed by the light scattering and turbidity experiments. From the results of the cloud point measurements, it is observed that the ATBN‐epoxy system showed upper critical solution temperature (UCST) behavior. From the results of the light scattering experiments, the domain correlation length increases as the content of ATBN incr